The present invention is in the field of colored and polarizing glasses and is concerned with the optical treatment of colored glasses containing both metallic silver and silver halide phases to produce colored and optically anisotropic glasses.
The coloration of a glass by the metals gold, silver and copper has been known since ancient times. "Ruby" glasses were investigated as early as 1857 by M. Faraday, who concluded that the coloration therein resulted from the presence of minute particles of gold within the glass. Similarly, metallic silver and copper particles were known to yield, respectively, yellow and red colored glasses.
The staining of glasses by metals such as silver and copper is also a relatively old technique. Essentially, such staining involves exposing the glass surface to metal ions at temperatures sufficient to cause ion migration into the glass, usually in exchange for alkali metal ions present therein. Exposure to copper and silver ions typically involves the use of a molten salt bath or a suitable compound of the metal mixed with a carrier which can be sprayed or painted on the glass surface.
If the oxidation state of the glass is sufficiently reduced, or if the glass includes suitable low temperature reducing agents such as Fe.sup.+2, Sb.sup.+3 or As.sup.+3, the exchanged copper or silver ions can be reduced to neutral atoms and caused to precipitate in the glass as colloidal metal particles. The precipitation process may or may not require a heat treatment step subsequent to the ion exchange treatment.
Stookey has recently demonstrated, in U.S. Pat. Nos. 2,515,936 and 2,515,943 that colloidal particles of silver, and gold may be photosensitively precipitated in glass to produce coloration. This process essentially involves photoreduction of atomic metal in selected volume regions of certain glasses, followed by heat treatment to develop the desired color.
In all of the above cases, precipitated colloidal metal particles are deemed to be the selective light-absorbing constituents which color the glass. In the case of metallic silver, the color observed is usually predominantly yellow or brown. The optical absorption of very small spherical silver colloids is characterized by a single, relatively sharp absorption peak in the violet, blue or green region of the visible spectrum. The exact location of the absorption peak, and the resulting glass color, depend primarily on the size of the silver colloids and upon the refractive index of the glass.
The conventional coloration effects of silver in glass can be considerably varied if, in addition to the glass matrix and the silver colloids, a third phase is present in the system. The observed effects are presumably due to the fact that the silver colloids in a three-phase system may be in or on the third phase as well as in the matrix glass. The existence of such systems and some of the properties observed therein are described by Forst and Kreidl in "Red Silver Glasses", Journal of the American Ceramic Society, Volume 25, Number 10, pages 278-280 (1942).
Another multi-phase system is described by W. H. Armistead in a copending, commonly assigned application, Ser. No. 715,989, filed Aug. 19, 1976, now U.S. Pat. No. 4,075,024. That system comprises silver-containing glasses exhibiting a wide range of transparent yellow, orange, red, blue and green colors. The glasses described are phase-separated glasses, and the wide range of colors observed therein is attributed to an unusual arrangement of silver particles at the interface between two glassy phases.
Certain of the unusual coloration effects found in silver-containing glasses may be due to the fact that the silver colloids present in the glass are non-spherical. The absorption behavior of non-spherical silver colloids is considerably more complex than that of spherical colloids. Thus the shape of the absorption curve of an elongated silver particle, such as an ellipsoid, depends not only on the size and degree of elongation of the particle, but also upon the nature of the incident light. Specifically, the absorbing properties of the particle depend on the degree of polarization of the incident light as well as the orientation of the particle with respect thereto.
To briefly review the phenomenon of light polarization, according to the wave theory, light is considered to propagate with its electric vector E perpendicular to the direction of propagation. Linearly polarized light is light whose characteristic E vector is oriented in a fixed direction in this perpendicular plane. The state of polarization of the light is characterized by the relationship of this fixed direction to some reference direction, e.g., vertical or horizontal polarization with respect to the horizon, or perpendicular or parallel polarization with respect to a given axis in the plane.
A beam of natural light consists of all polarizations. That is, the direction of the E vector varies randomly in the plane perpendicular to the direction of propagation. At any instant the E vector may be resolved into components perpendicular and parallel to a selected reference direction in this plane. If one of these components is selectively absorbed as the beam of light passes through a medium, the light that is transmitted through the medium is considered to be linearly polarized. A medium having this property of selective absorption is called a polarizer.
The more common polarizers are composed of plastics, but glasses which linearly polarize light are also known. These glasses are referred to as dichroic glasses, the term dichroic referring in this sense to the optical anisotropy of the glass with respect to its absorption coefficient .alpha.. Thus the absorption of light passing through such a glass depends upon the direction of polarization of the light with respect to the glass.
Taking the specific case of light absorption by an ellipsoidal (prolate or oblate) metallic silver particle as an example, if the incident light is linearly polarized such that its E vector is parallel to the long axis of such a particle, the absorption maximum is shifted to a longer wavelength than for a corresponding spherical particle. On the other hand, the absorption maximum for light polarized with its E vector perpendicular to the long axis of the particle is shifted to shorter wavelengths. The extent of these shifts increases with the degree of elongation of the particle.
For a glass containing ellipsoidal metallic silver particles which are randomly oriented in space, the absorption curve exhibited by the glass would correspond to a weighted average of the absorption curves exhibited by a single particle taken over all possible orientations with respect to a beam of linearly polarized light. Hence the glass would be optically isotropic.
For a glass wherein the ellipsoidal metallic particles are aligned in a common direction, however, the absorption behavior of the glass is analogous to that of a single particle. Hence, a singly- or doubly-peaked absorption curve will be observed depending upon the degree of polarization of the incident light and the direction of polarization of that light with respect to the direction of alignment of the particles in the glass. The fact that, for polarized light, absorption varies considerably depending upon the direction of polarization, means that the glass is dichroic and can act as a polarizing medium.
The dichroic properties of stretched glasses containing elongated silver particles approximating prolate ellipsoids have been described by Stookey and Araujo in "Selective Polarization of Light Due to Absorption by Small Elongated Silver Particles in Glass", Applied Optics, Volume 7, Number 5, pages 777-779 (1968). Similar properties in stretched glasses comprising lead or gold have been reported by Land in U.S. Pat. No. 2,319,816.
Other glasses wherein metallic silver fulfills an important absorbing function are so-called photochromic glasses containing silver halides. These glasses, described by Armistead and Stookey in U.S. Pat. No. 3,208,860, comprise submicroscopic crystals of a silver halide such as silver chloride, silver bromide or silver iodide, and are typically colorless in the unactivated state. However, upon exposure to actinic radiation such as ultraviolet light, the glass darkens (becomes more absorbing with respect to visible light) to an extent which depends somewhat on the intensity of the actinic radiation employed. Upon termination of this radiation, the glasses return to the clear, non-absorbing state.
The behavior of these glasses is explained in terms of the photolytic reduction of silver ions in the silver halide crystals to metallic silver through the action of ultraviolet light. Specks of silver which absorb visible light are formed at the crystal sites. However, since the products of the photolytic reduction are trapped at these sites by the matrix glass, recombination to silver halides occurs when exposure to the actinic radiation is terminated.
Although the color of these photochromic glasses in the darkened state may vary, they are not generally colored in the clear or unactivated state. Thus recombination of metallic silver and halogen to form silver halide can be complete, whereupon no detectable residual metallic silver remains at the crystal sites to color the glass.
It is known that photochromic glasses of the silver halide type are bleachable to varying degrees by the action of visible light. That is, the conversion of these glasses from the darkened state (induced by irradiation with ultraviolet light) to the original clear, non-light absorbing state is accelerated by exposure to certain wavelengths of visible light. U.S. Pat. No. 3,630,765 to Araujo describes optically-bleachable photochromic glasses which are bleached by the action of red or near infrared radiation.
A family of silver-halide containing photochromic glasses exhibiting unusual darkening effects is disclosed by Randall and Seward in U.S. Pat. No. 3,734,754. Seward notes, in the Journal of Applied Physics, 46, 689 (1975), that red and purple colors are observed in these glasses which can be optically bleached, but redeveloped upon heating the glass. The coloration is tentatively attributed to the presence of silver metal in contact with the silver halide photochromic phase.
The use of polarized optical bleaching light to modify the light absorbing and light refracting properties of silver chloride photographic emulsions, sols and single crystals which have been chemically or optically darkened is described by Cameron and Taylor in "Photophysical Changes in Silver-Silver Chloride Systems", Journal of the Optical Society of America, 24, 316-330 (1934). These authors observed that two different effects were produced by irradiation with polarized bleaching light. The first effect, hereinafter referred to as photo-adaptation, involved a change in color caused by the action of the irradiating light, such that the photo-adapted material tended to assume the color of the light by which it was produced.
A second effect, hereinafter referred to as photoanisotropy, involved the appearance of anisotropic effects which included dichroism and birefringence. These are termed photo-dichroism and photo-birefringence.
For the purposes of the following description, the term photo-coloration will be employed to refer to all changes in color which occur upon irradiating a material with light, whether or not the coloration which results approximates that of the irradiating light. Hence, materials which exhibit coloration differing from that of the irradiating light will be discussed, as will materials exhibiting all of the so-called photo-alteration or optical-alteration effects above described.
The copending commonly-assigned application of Araujo et al., Ser. No. 739,122, concurrently filed herewith, describes photo-alteration treatments which can be used to modify the light absorbing and refracting properties of silver halide-containing photochromic glasses. In general, useful properties are obtained in accordance with those treatments by optically bleaching a photochromic glass while it is in an at least partially darkened (light-absorbing) state.